Oil fired boilers have long been used to supply hot water for hydronic heating. The hydronic heating can entail both the heating of an occupied space and for domestic hot water. These types of systems typically use separate oil burner controls and boiler water temperature limit controls.
A typical oil burner fired boiler can be controlled by an oil burner control such as a control manufactured by Honeywell and identified as an R4184 or an R8184. This type of a control utilizes a cadnium sulfide flame sensor to respond to a flame in an oil burner. This control typically would be responsive to a room thermostat and would contain its own step-down or power transformer.
Used in conjunction with an oil burner control for hydronic space heating there is usually provided a boiler water temperature limit control such as that sold by Honeywell and identified as an L8148. This type of control is referred to in the trade as an Aquastat Relay. This control typically contains its own power transformer and related circuitry.
Where a system involves domestic hot water, the system typically would use a more complex boiler water temperature limit control which has two sets of contacts. This type of control is sold by Honeywell as an L8124 and is known as a triple Aquastat Relay. This control contains its own transformer and circuitry, along with high-limit contacts and a pair of low-limit contacts. The high-limit contacts protect the boiler against excessive boiler water temperatures, while the low-limit contacts are used to insure that the water temperature in the boiler is never allowed to drop below a level needed to supply the domestic hot water.
The combination of an oil burner control and a boiler water temperature limit control works quite well, but is very large and expensive. The size and expense is dictated by the fact that each of the controls is independent of one another, and contain their own power transformers and circuitry. This type of equipment is becoming more and more difficult to apply to specific boiler applications due to the cost of two separate units and the physical space that the two units require. For these reasons, this type of equipment, while functionally adequate, is less and less competitive.